1. Field of the Invention
The present invention relates to a charge equalizing device for a power storage unit. More specifically, the present invention relates to a charge equalizing device which equalizes disparities in the state of charge (SOC; the remaining charge) among the secondary cells of a power storage unit that includes a plurality of secondary cells which are connected in series or series-parallel.
2. Description of Related Art
In an electrical vehicle (EV) or a hybrid electrical vehicle (HEV), it is generally required to use a high voltage battery in which a plurality of battery cells (hereinafter also referred to as cells) are connected in series. If such a battery is used for a certain period of time while repeatedly discharging and recharging the battery, or if it is left unused for a long time, disparities in the discharging-recharging efficiency of each cell forming the battery may occur. Also, depending on the differences in the environmental temperature to which the cells are exposed, disparities in the remaining charge (hereinafter also referred to as SOC disparities) of each cell may arise.
As for the charging and discharging processes for the battery, it is necessary, from the viewpoint of durability of each cell or for protection of the battery, to stop the charging process when the SOC value (or cell voltage) of the cell having the highest SOC value (or cell voltage) at that moment reaches the predetermined upper limit, and to terminate the discharging process when the SOC value (or cell voltage) of the cell having the lowest SOC value (or cell voltage) at that moment reaches the predetermined lower limit. Therefore, the usable charge of the battery is substantially decreased if disparities in the charging capacities of individual cells occur. For this reason, a so-called assist-regeneration process in which battery energy supplements gasoline power on uphill stretches and energy is regenerated for the battery on downhill stretches becomes insufficient in HEV""s. Accordingly, the actual vehicle power performance decreases or the fuel consumption of the vehicle increases.
In order to solve the above-mentioned problems, a means for equalizing the SOC disparities of each cell and ensuring a suitable usable charging capacity of the battery becomes necessary. Moreover, for an energy storage member, such as a lithium ion battery or a super-capacitor, in which the charging-discharging efficiency does not change until it enters its overcharge region, an additional system for carrying out an equalizing process becomes essential.
As a method for performing the equalizing process for each cell, a so-called bypass circuit method in which a bypass circuit having a voltage sensor, a bypass resistance, and a bypass switch is provided for each cell and the bypass switch is controlled by using a microcomputer, has been proposed and actually implemented for vehicles such as HEV""s.
FIG. 10 is a structural diagram showing a conventional bypass circuit which is generally used. In this system, SOC values calculated from voltage values of cells 21a, 21b, . . . measured by voltage sensors 22a, 22b, . . . are mutually compared and then, for instance, the cell 21a which, in this case, is determined to have a high SOC value is discharged by turning on a bypass switch 23a for a certain period of time calculated from the SOC value. In this manner, the disparity of SOC values in each of cells 21a, 21b, . . . is equalized.
However, in the conventional bypass circuit, voltage sensors 22a, 22b, . . . must be provided for each of the cells 21a, 21b, . . . to carry out the equalization of the SOC values of the cells 21a, 21b, . . . Also, the calculation of the SOC value for each of the cells 21a, 21b, . . . based on the measured voltage values becomes necessary. Moreover, since it becomes necessary to carry out an ON/OFF control of the bypass switch 23a, 23b, . . . corresponding to the cell 21a, 21b, . . . based on the calculated SOC values, the bypass circuit becomes complicated and expensive.
In order to solve the above mentioned problems, for instance, a method for equalizing the disparities in remaining charge of each cell has been proposed in which cells forming the battery are connected in parallel so that electric charge is moved from cells having a large remaining charge to cells having a small remaining charge.
According to the above method, since only a switching element for connecting cells in parallel is necessary and a complicated bypass circuit becomes unnecessary, the system may be constructed at a significantly lower cost than that of the conventional method.
However, in the above-mentioned remaining charge equalizing method, there is a possibility that a large amount of current may flow locally in the battery if the switching element for connecting the cells in parallel is stuck, and this may cause problems such as a deterioration of the battery.
Accordingly, an object of the invention is to provide a charge equalizing device for a power storage unit having a simple structure at a low cost, which is capable of equalizing the disparity of SOC among the cells by selecting a suitable combination of connections among the cells.
Also, another object of the invention is to provide a charge equalizing device for a power storage unit having a simple structure manufacturable at a low cost, which is capable of equalizing the disparity of SOC values among the cells by suitably connecting the cells in parallel, and avoiding the deterioration of the battery by quickly detecting such problems as a stuck state of the switches used for connecting the cells in parallel and preventing the occurrence of overcurrents.
Accordingly, the present invention provides a charge equalizing device for a power storage unit, which equalizes the remaining charge of each cell unit of the power storage unit that includes a plurality of groups of cells formed by a plurality of the cell units which are connected in series, including: a connection device (the main switch 1 in an embodiment) provided with (for instance, between the group A cells and group B cells in one embodiment) a series connection circuit for the groups of cells, the connection device being capable of connecting and disconnecting the groups of cells; a connection terminal provided for each of the cell units; and a switch device (the cell parallel connection switches 4 in one embodiment) which connects one of the connection terminals of one of the groups of cells to another connection terminal of another group of cells, wherein the switch device is capable of changing the combination of the connection terminals to be connected.
According to the above charge equalizing device, unlike conventional bypass circuit methods in which the equalizing process is carried out by consuming the energy of a cell having a high remaining charge by converting it to heat by using the bypass resistance, the energy is not consumed as heat during the equalizing process and may be efficiently used since the charge is transferred from cells having a high remaining charge to cells having a low remaining charge.
In accordance with another aspect of the invention, the charge equalizing device further includes: an overcurrent blocking device (a fuse 5 in one embodiment) provided with a closed circuit which is formed when the switch device is turned on, the overcurrent blocking device preventing an overcurrent from flowing in the closed circuit.
According to the above charge equalizing device, the generation of an excessive rush current immediately after the switch devices are turned on may be prevented. It is preferable to provide a rush current inhibition resistance for each parallel circuit of the cell units.
Also, the power storage unit including the charge equalizing device of the present invention may be used for an electric vehicle or a hybrid vehicle, and the charge equalizing device equalizes the remaining charges of a plurality of cell units which form the power storage unit (e.g., battery, capacitor, etc.,) by switching on or off a cell parallel connection means after the connection device is disconnected when the ignition is turned off.
The present invention also provides a charge equalizing device for a power storage unit, which equalizes the remaining charge of each cell unit of the power storage unit that includes a plurality of groups of cells formed by a plurality of the cell units which are connected in series, including: a connection device (the main switch 1 in one embodiment) provided with a series connection circuit of the groups of cells (for instance, between the group A cells and the group B cells in one embodiment), the connection device being capable of connecting and disconnecting the groups of cells; a connection terminal provided for each of the cell units; a switch device (the cell parallel connection switches 4 in one embodiment) which connects one of the connection terminals of one of the groups of cells to the other one of the connection terminals of another group of cells; and a control unit (the control unit 20 in an embodiment) which controls the connection of the switch device, wherein the control unit, after disconnecting (i.e., turning off) the connection device and confirming that all of the switch devices have been turned off, starts controlling the switch devices.
According to the above charge equalizing device, problems such as a switch device being stuck may be quickly detected, and the flow of an overcurrent in a part of the power storage unit may be prevented.
In accordance with another aspect of the invention, the control unit is capable of changing the combination of connection terminals to be connected (in one embodiment, the group A parallel connection switches 4a and the group B parallel connection switches 4b are alternately connected).
In yet another aspect of the invention, the control unit, after turning off the switch device connected at that moment and confirming that all of the switch devices have been turned off, turns on the switch devices.
In this manner, problems such as a switch device being stuck may be quickly detected since the switch device is turned on for a new combination of cell units after confirming that all of the switch devices have been turned off.
In yet another aspect of the invention, the control unit confirms that all of the switch devices have been turned off by confirming that there is no voltage difference between both ends of the power storage unit (in an embodiment, the voltage between the electric power extracting terminals is confirmed to be zero volts).
According to the above charge equalizing device, problems such as a switch device being stuck may be easily detected without providing a complex problem handling function.
In yet another aspect of the invention, the above charge equalizing device further includes: an overcurrent blocking device (a fuse 5 in an embodiment) provided with a closed circuit which is formed when the switch device is turned on, the overcurrent blocking device preventing an overcurrent from flowing in the closed circuit.
According to the above charge equalizing device, the generation of an excessive rush current immediately after the switch devices are turned on may be prevented. It is preferable to provide a rush current inhibition resistance for each parallel circuit of the cell units.
The present invention also provides a method for equalizing the remaining charge of each cell unit of a power storage unit that includes a plurality of groups of cells formed by a plurality of the cell units which are connected in series using the above charge equalizing device including the steps of: disconnecting the connecting device; determining the number of times to repeat turning the switch device on and off based on the difference in voltage between the groups of cells; turning on the switch device to carry out an equalizing process for the cell units connected by the switch device via the respective connection terminal; changing the combination of cell units connected by the switch device and turning on the switch device to carry out an equalizing process for the cell units connected by the switch device via the respective connection terminal; and confirming that the voltage difference between the groups of cells is within a predetermined value.
In accordance with another aspect of the invention, the method further includes the step of: determining whether the switch device has been turned on and off a predetermined number of times.
In yet another aspect of the invention, the method further includes the step of: determining if the voltage of the power storage unit is 0 V.
In yet another aspect of the invention, the method further includes the step of: issuing a notification of system failure if it is determined that the voltage of the power storage unit is not 0 V.